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2767 Dewey Avenue
Rochester, NY 14616
Office: 585-349-3900
Fax: 585-349-3834

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Background Information on Radon

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Radon is a naturally occurring, chemically inert, radioactive gas. Because radon is chemically unreactive with most materials, it is free to travel as a gas. It can move easily through very small spaces such as those between particles of soil and rock. Radon is odorless, invisible, and without taste; thus, it cannot be detected with the human senses. Radon is also moderately soluble in water and, therefore, can be absorbed by water flowing through rock or sand. Its solubility depends on the water temperature; the colder the water, the greater radon's solubility.

The two natural sources of radon, thorium and uranium, are common, naturally occurring elements that are found in low concentrations in rock and soil. Through radioactive decay, both are constant sources of radon. Radon is produced from the radioactive decay of the element radium, which is itself a decay product of either uranium or thorium. Radioactive decay is a process in which an unstable atomic nucleus undergoes spontaneous transformation, by emission of particles or electromagnetic radiation, to form a new nucleus (decay product), which may or may not be radioactive. The level of radioactivity is measured in curies, where 1 curie equals 37 billion disintegrations per second. The time required for a given specific activity of an isotope to be reduced by a factor of two is called its half-life. A Picocuries (pCi) is equal to one-trillionth of a curie. Specific activity concentrations are typically measured in Picocuries per gram (in a solid) or Picocuries per liter (in a gas, such as air).

Uranium-238 decays in several steps to radium-226, which decays into radon-222. Radon-222 has a half-life of 3.8 days and, therefore, has enough time to diffuse through dry, porous soils or to be transported in water for a considerable distance before it decays. Similarly, thorium-232 decays into radon-220 (a different radon isotope, also called thoron), which has a half-life of only 55 seconds. Because of its short half-life and limited ability to migrate into residences, radon-220 is usually a less important source of radon exposure to humans. The United Nations Scientific Committee on the Effects of Atomic Radiation has estimated the average exposure from indoor radon-220 decay products to be about 25 percent of that from radon-222. Only radon-222 is addressed specifically in the Citizen's Guide because it is the radon isotope of most concern to the public. Although radon-220, or thoron, has not been measured separately in most homes, radon mitigation will also reduce exposure to thoron.

Radon-222 is preceded in the uranium-238 decay series by radium-226, which has a half-life of 1,600 years. Radon-222 decays in several steps to form radioactive isotopes with short half-lives: Polonium-218, lead-214, bismuth-214, and polonium-214. These isotope particles are commonly referred to as radon decay products, daughters, or progeny. Radon decay products are chemically reactive and can attach themselves to walls, floors, or airborne particles that are inhaled into the lungs. Unattached radon decay products also can be inhaled and, subsequently, can become deposited on lung tissue.

The four radon-222 decay products just mentioned all have half-lives of less than 30 minutes. This short half-life is significant since, once deposited on lung tissue, the radon decay products can undergo considerable decay before the action of mucus in the bronchial tubes can clear these radioactive particles. Two of the short-lived decay products, polonium-218 and polonium-214, emit alpha particles during the decay process. An alpha particle is a subatomic particle that has two protons and two neutrons and has a double positive electrical charge. It is identical to a helium nucleus.

Radon-222 is found virtually everywhere in at least small amounts because its predecessor, radium-226 (or, more distantly, uranium-238), is found in all rock and soil. In outdoor air, radon concentrations are usually less than one Picocuries per liter (pCi/L). Higher concentrates of radon outdoors may be observed during brief periods, such as during a temperature inversion, when a warm air mass traps a colder one beneath it. The highest individual concentration, in contrast, can vary from around 0.5 pCi/L to over 2,000 pCi/L, with results from EPA's National Residential Radon Survey indicating that over 6 percent of all homes nationwide have average annual indoor radon levels above 4 pCi/L. Most indoor radon comes from rocks and soil around a home, although other, usually less significant, sources of indoor radon are water and some construction materials.


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Voice 585-349-3900 - Fax 585-349-3834
2767 Dewey Avenue - Rochester, NY 14616 -
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